Nj Vacation and custody/visitation question? New Jersey?

I have custody of my niece and nephew and their natural parents have visitations on Saturdays. My nephew (14) wants to go with his grandfather on vacation, missing a Saturday visitiation with his parents. My nephew and his grandfather both got the ok from the parents. Neither parent has talked to me about it directly, however, because our relationship is strained due to their drug/alcohol abuse. I am the parent of primary residence and they have court-ordered visitation every Saturday. Do I need to get something from the natural parents in writing that they waive their visitation that day? Should I go to the courthouse for approval if they refuse? I'm worried that they will call the police on me on that day, even though they verbally approved the change--- just not to me. The trip is in about a week. Thanks